So, you're a newly minted assistant professor. You want to write a good grant proposal. I have some advice, subject to your approval.
First, take any and all opportunities to sit on funding panels. NSF is a good place to start. E-mail program managers in your field(s) of study, and let them know that you're available for public service in reviewing grant proposals. Look deep into the proposals that you read, and ask yourself about various forms of merit. As a reviewer, what do you want to see in a good proposal? Chances are, you'll see some stellar proposals, and some horrible proposals, and a lot in between. Then, you'll get to sit in on a group discussion about all of them, and you'll get to know both sides of the process, submitting and reviewing. Some of the discussions will be inexplicable and probably political, but there will also be a great deal of real assessment of merit, and you will be part of those discussions.
Then, consider writing a proposal from the perspective of being a reviewer. I can't stress enough how important this experience can be for you. Now that you've read that variety of proposals, and heard what others have to say, you are much more well-positioned to write one that you-as-reviewer would be proud to vote to the top of the heap.
This may surprise you, but NSF does not require that a reviewer is a professor, or even that they have a doctorate in the topic of review. All that is required is that you are highly knowledgeable in that field of discourse, and that you are willing to perform this kind of public service.
It also looks very good on your curriculum vitae.